meteor/packages/appcache/appcache-client.js

import { Meteor } from 'meteor/meteor';

if (window.applicationCache) {

  const appCacheStatuses = [
    'uncached',
    'idle',
    'checking',
    'downloading',
    'updateready',
    'obsolete'
  ];

  let updatingAppcache = false;
  let reloadRetry = null;
  let appcacheUpdated = false;

  Reload._onMigrate('appcache', retry => {
    if (appcacheUpdated)
      return [true];

    // An uncached application (one that does not have a manifest) cannot
    // be updated.
    if (window.applicationCache.status === window.applicationCache.UNCACHED)
      return [true];

    if (!updatingAppcache) {
      try {
        window.applicationCache.update();
      } catch (e) {
        Meteor._debug('applicationCache update error', e);
        // There's no point in delaying the reload if we can't update the cache.
        return [true];
      }
      updatingAppcache = true;
    }

    // Delay migration until the app cache has been updated.
    reloadRetry = retry;
    return false;
  });

  // If we're migrating and the app cache is now up to date, signal that
  // we're now ready to migrate.
  const cacheIsNowUpToDate = () => {
    if (!updatingAppcache)
      return;
    appcacheUpdated = true;
    reloadRetry();
  };

  window.applicationCache.addEventListener('updateready', cacheIsNowUpToDate, false);
  window.applicationCache.addEventListener('noupdate', cacheIsNowUpToDate, false);

  // We'll get the obsolete event on a 404 fetching the app.manifest:
  // we had previously been running with an app cache, but the app
  // cache has now been disabled or the appcache package removed.
  // Reload to get the new non-cached code.

  window.applicationCache.addEventListener('obsolete', () => {
    if (reloadRetry) {
      cacheIsNowUpToDate();
    } else {
      appcacheUpdated = true;
      Reload._reload();
    }
  }, false);

} // if window.applicationCache